Cerium-containing oxide nanoparticles have many current industrial uses, as well as many potential applications in the future. They are well known as important components in solid oxide fuel cells, three-way automotive exhaust catalysts, automotive fuel borne catalysts, and ultra-violet sun blockers, to name just a few. Its utility is often attributed to its solid state redox chemistry, resulting from the relatively facile Ce3+/Ce4+ electrochemical conversion. This allows nanoceria, for example, to store oxygen under oxidizing conditions, wherein Ce3+ is converted to Ce4+, and to release oxygen under reducing conditions, wherein Ce4+ is converted to Ce3+ and oxygen vacancies are created, a property commonly referred to as its oxygen storage capacity (OSC). As an automotive fuel borne catalyst, the ability of nanoceria to store and release oxygen in a diesel engine combustion chamber, whereby local inhomogeneities in the fuel/oxygen mixture are reduced, is believed to produce a more complete burn, thereby generating more power with reduced soot and toxic gas emissions.
Many of these end use applications benefit directly from the small particle size of nanoceria due to increased surface area and enhanced reactivity. There are many synthetic methods for the production of metal oxides, including aqueous or organic precipitation, hydrothermal precipitation, spray precipitation, chemical vapor deposition, and plasma deposition techniques. Aqueous precipitation methods are particularly favored in cases where high through-put is desired, wherein a relatively large amount of product is to be produced. However, conventional metal oxide precipitation processes typically include the multiple steps of reactant delivery, particle precipitation, isolation, washing, drying, impregnation, calcination (heating to 400-1000° C. for several hours), grinding, milling and particle size classification, among others. Alternatively, direct methods seek to produce a dispersion (suspension) of the final particles directly, thereby avoiding the time, cost and potential contamination inherent in the isolation, drying, calcination, grinding, milling and classification steps. For many end use applications, however, these direct methods present the additional challenge of maintaining dispersion stability (preventing aggregation or clumping of particles) during subsequent washing, handling and storage of the dispersed product particles.
Aqueous precipitation methods for the direct preparation of nanoceria are described in U.S. Pat. No. 5,389,352; U.S. Pat. No. 5,938,837 and U.S. Patent Appl. No. 2007/0215378. The basic precipitation process described in these references involves adding a cerium (III) salt and a base, such as ammonium hydroxide, and converting the cerium (III) salt into a ceria (CeO2) precipitate. In some cases an oxidant, such as hydrogen peroxide (H2O2) was also included.
Wang, U.S. Pat. No. 5,389,352, describes the reaction of cerous nitrate with ammonia at high temperatures (above 100° C.) in a closed container for 24 hours. These hydrothermal precipitations produce a slurry of ceria, evidence of the instability of the particle dispersions. Alternatively, a room temperature reaction of H2O2, cerous nitrate and ammonia over a 4 hours period is described as producing a powder with average crystallite size of about 7 nanometers (nm). However, there is no description of the actual agglomerated particle size, as would be revealed by a transmission electron microscopy (TEM) analysis, or a hydrodynamic diameter measurement by a dynamic light scattering technique. There is also no teaching of the use of a stabilizer additive to improve dispersion stability, nor any suggestion of how to reduce the time of the reaction.
Hanawa, U.S. Pat. No. 5,938,837, describes the precipitation of ceria from an aqueous solution based reaction of cerous nitrate and ammonia at a pH range between 5 and 10, preferably between 7 and 9, along with the use of a carefully timed temperature ramp up to 70-100° C. within 10 minutes of initial mixing of the reactants. It is evident that these particle dispersions have very poor stability as a slurry of particles is produced. While a crystallite size of about 20 nm was determined from X-ray Diffraction peak widths and confirmed by TEM analysis, the particles are highly agglomerated as evidence by the TEM image of FIG. 2, which was taken after a deagglomeration step. There is no teaching of the use of an oxidant, nor any suggestion to employ a stabilizer additive to reduce the particle agglomeration or to improve the dispersion stability.
Zhou et al., U.S. Pat. Appl. 2003/0215378, describes the aqueous precipitation of slurries of cerium dioxide resulting from the reaction of cerium nitrate and ammonium hydroxide during which oxygen is bubbled through the reaction mixture. The basic process followed is to form a precipitate, and then to filter and dry the precipitate. While the primary crystallite sizes are quite small (3-100 nm), the particles are substantially aggregated as shown in TEM images taken only after the samples were prepared by ultrasonically dispersing the powder in ethanol. There is no suggestion to employ an oxidant stronger than molecular oxygen. There is no suggestion to employ a stabilizer additive to reduce the particle aggregation or to improve the particle dispersion stability.
Cuif et al., U.S. Pat. No. 6,133,194, describes the use of anionic surfactants, non-ionic surfactants, polyethylene glycols, carboxylic acids, and carboxylate salts as additives in a conventional aqueous precipitation or co-precipitation process involving cerium solutions, zirconium solutions, base and optionally an oxidizing agent, at a pH preferably greater than about 7, wherein after the reaction stage, mixed hydroxides, such as (Ce,Zr)(OH)4, are precipitated, the solid precipitate is recovered and separated from the mother liquor by conventional solid/liquid separation techniques such as decantation, drying, filtration and/or centrifugation, then washed, calcined at a minimum temperature of 400° C., a temperature high enough to ensure removal of carbonaceous remnants from the oxide, hydroxide or carbonate. Many additives are disclosed for addition to the reaction mixture from which the mixed hydroxides are precipitated, isolated, washed and calcined. Many alkoxylated compounds are disclosed for use in the washing or impregnation, preferably in the form of a wet cake, followed by calcination. There is no disclosure of monoether carboxylic acids, or salts thereof, as an additive. Furthermore there is no suggestion to use any of the additives disclosed therein in a direct preparation method of making metal oxide, hydroxide or carbonate particles with a goal of reducing particle size or maintaining or improving particle dispersion stability.
Poncelet et al., FR 2885308, describe the use of polyether carboxylic acids (2-(2-methoxyethoxy) acetic acid (MEAA) and 2-(2-(2-methoxyethoxy)ethoxy) acetic acid (MEEAA)) and the monoether carboxylic acid (3-methoxypropionic acid (MPA)) as an additive in the preparation of method of Cuif et al. (U.S. Pat. No. 6,133,194) for cerium oxide, zirconium oxide or a mixed oxide of cerium and zirconium. Example 3 shows that use of a specific monoether carboxylic acid, 3-methoxypropionic acid (MPA), in the preparation of cerium oxide Ammonium hydroxide is added to a solution containing a mixture of MPA and cerous nitrate in the molar ratio of 0.16 MPA to cerium ion. The resulting product was a suspension (dispersion) of cerium oxide particles. The size of the aggregates formed is reported as having a hydrodynamic diameter of 50-60 nm. Furthermore, the specification clearly states that the alkoxy carboxylic acid/metallic oxide molar ratio is between 0.01 and 0.2. Preferably the alkoxy carboxylic acid/metallic oxide ratio is between 0.05 and 0.15. There is no suggestion in Poncelet et al. (FR 2885308) to employ an oxidant additive such as hydrogen peroxide.
There is a need to provide small nanoparticles of metal oxides, such as cerium oxides and homogeneously doped cerium oxides, and to provide robust, cost-effective methods for their preparation. To date, the smallest aggregate size achieved using a monoether carboxylic acid stabilizer in an aqueous preparation of cerium oxide is only 50-60 nm. There is a need to provide aqueous dispersions of metal oxide nanoparticles, such as cerium oxides and homogeneously doped cerium oxides, with excellent dispersion stability, particularly when the polarity of the solvent is reduced to improve the compatibility of the dispersion with a hydrocarbon diluent/fuel, such as kerosene, diesel fuel or biodiesel fuel. There is a need to provide fuel additives with improved fuel efficiency, reduced toxic gas and particulate emissions, and reduced engine conditioning time before the benefits of the fuel additive are realized.